BASIC RESEARCH www.jasn.org Chronic Hyponatremia Causes Neurologic and Psychologic Impairments † Haruki Fujisawa,* Yoshihisa Sugimura,* Hiroshi Takagi,* Hiroyuki Mizoguchi, ‡ Hideyuki Takeuchi, Hisakazu Izumida,* Kohtaro Nakashima,* Hiroshi Ochiai,* † | Seiji Takeuchi,* Atsushi Kiyota,* Kazuya Fukumoto, Shintaro Iwama,*§ Yoshiko Takagishi, | | Yoshitaka Hayashi, Hiroshi Arima,* Yukio Komatsu,¶ Yoshiharu Murata, and Yutaka Oiso* *Department of Endocrinology and Diabetes, Nagoya University Graduate School of Medicine, Nagoya, Japan; †Futuristic Environmental Simulation Center, Research Institute of Environmental Medicine, Nagoya University, Nagoya, Japan; ‡Department of Neuroimmunology, Research Institute of Environmental Medicine, Nagoya University, Nagoya, Japan; §Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, | Japan; Department of Genetics, Research Institute of Environmental Medicine, Nagoya University, Nagoya, Japan; and ¶Department of Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Nagoya, Japan ABSTRACT Hyponatremia is the most common clinical electrolyte disorder. Once thought to be asymptomatic in response to adaptation by the brain, recent evidence suggests that chronic hyponatremia may be linked to attention deficits, gait disturbances, risk of falls, and cognitive impairments. Such neurologic defects are associated with a reduction in quality of life and may be a significant cause of mortality. However, because underlying diseases such as adrenal insufficiency, heart failure, liver cirrhosis, and cancer may also affect brain function, the contribution of hyponatremia alone to neurologic manifestations and the underlying mecha- nisms remain unclear. Using a syndrome of inappropriate secretion of antidiuretic hormone rat model, we show here that sustained reduction of serum sodium ion concentration induced gait disturbances; facilitated the extinction of a contextual fear memory; caused cognitive impairment in a novel object recognition test; and impaired long-term potentiation at hippocampal CA3–CA1 synapses. In vivo microdialysis revealed an elevated extracellular glutamate concentration in the hippocampus of chronically hyponatremic rats. A sus- tained low extracellular sodium ion concentration also decreased glutamate uptake by primary astrocyte cultures, suggesting an underlying mechanism of impaired long-term potentiation. Furthermore, gait and memory performances of corrected hyponatremic rats were equivalent to those of control rats. Thus, these results suggest chronic hyponatremia in humans may cause gait disturbance and cognitive impairment, but these abnormalities are reversible and careful correction of this condition may improve quality of life and reduce mortality. J Am Soc Nephrol 27: 766–780, 2016. doi: 10.1681/ASN.2014121196 Hyponatremia is the most common clinical elec- attention deficits,6 gait disturbances,6 ariskof trolyte disorder.1,2 Symptoms of hyponatremia fracture associated with falls,7,8 and cognitive depend chiefly on its magnitude and rapidity of onset.3 Acute hyponatremia can cause neurologic Received December 9, 2014. Accepted May 12, 2015. complications and death as a result of osmotically Published online ahead of print. Publication date available at 4 induced cerebral edema. On the other hand, www.jasn.org. chronic hyponatremia has been considered Correspondence: Dr. Yoshihisa Sugimura, Department of En- asymptomatic because the brain can successfully docrinology and Diabetes, Nagoya University Graduate School adapt to hyponatremia that is associated with of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466-8550, hypo-osmolarity.5 However, recent evidence sug- Japan. Email: [email protected] gests that chronic hyponatremia may be linked to Copyright © 2016 by the American Society of Nephrology 766 ISSN : 1046-6673/2703-766 J Am Soc Nephrol 27: 766–780, 2016 www.jasn.org BASIC RESEARCH impairments.9 Furthermore, in Study of Ascending Levels of RESULTS Tolvaptan in Hyponatremia (SALT)-1 and SALT-2 trials, cor- recting chronic hyponatremia improved self-assessed mental Induction of Chronic Hyponatremia health status.10 These neurologic impairments are associated Continuous injections of the vasopressin V2 receptor agonist, with a reduction of quality of life and may be a significant 1-deamino-8-D-arginine vasopressin (dDAVP) and liquid diet cause of mortality. However, because underlying diseases feeding were used to induce hyponatremia in rats. A decreased such as adrenal insufficiency, heart failure, liver cirrhosis, serum [Na+]of121.3360.93 mEq/l in the moderate and cancer may affect brain function, the contribution of hyponatremia group (dDAVP was administered at a rate of hyponatremia itself to neurologic manifestations remains 0.3 ng/h) and 111.3361.54 mEq/l in the severe hyponatremia unknown. group (dDAVP was administered at a rate of 0.7 ng/h), as Brain cells can adapt to hyponatremia. After an acute measured on day 6 after the start of dDAVP injections, was noted decrease in external osmolality, cells will initially swell, as a (Figure 1, A and B). Body weight was comparable among the result of water movement into the cells along an osmotic three groups (Figure 1C). We measured the apparent diffusion gradient. Very soon thereafter, a process known as volume coefficient (ADC) using magnetic resonance imaging (MRI) to regulatory decrease begins, in which intracellular solutes rule out brain cell swelling in chronically hyponatremic rats. (electrolytes and organic osmolytes) are extruded together MRI analysis revealed no significant difference in ADC between with osmotically obligated water.5 Glutamate, a known major control and chronically, severely hyponatremic rats, whereas the excitatory neurotransmitter, is one such organic osmolyte that ADC of acutely hyponatremic rats (a serum [Na+] of 113.256 is extruded into the extracellular space during cellular adap- 1.21 mEq/l) was significantly decreased as compared with the tation to hyponatremia.5 In fact, under acute hypo-osmotic former two groups (Figure 1D). This meant that the brain water conditions, the brain’s extracellular glutamate concentration content of chronically hyponatremic animals could not be distin- is increased.11 guished from that of controls with the methods used. In a chronically hyponatremic state, as a result of adaptation to hyponatremia, brain volume normalizes Chronic Hyponatremia Induces Gait Disturbances completely.12 However, the brain content of glutamate re- To investigate the influence of chronic hyponatremia on gait in portedly decreases by 38.6% after 14 days’ sustained hypo- detail, we performed CatWalk automated gait analysis.14 As natremia in rats,12 which suggests that synaptic excitatory seen in the movie (Supplemental data), moderately or severely neurotransmissions are affected by chronic hyponatremia.5 hyponatremic rats with chronic disease seemed to walk with The effect of chronic hyponatremia on neurotransmission, larger alternating, lateral trunk movements. Footprint images however, remains unknown. Furthermore, the extracellular suggested moderately and severely hyponatremic rats had a glutamate concentration in a chronically hyponatremic smaller stride, wider-based stance, and separated front and state remains unquantified. The extracellular glutamate hind paw prints as compared with control rats (Figure 2A). concentration must be kept low to maintain a high signal- Quantitative analysis of footprint patterns showed moderately to-noise ratio in synaptic and extrasynaptic transmissions and severely hyponatremic rats displayed a significantly and to prevent glutamate neurotoxicity resulting from ex- shorter stride length (Figure 2B) and a significantly wider cessive activation of glutamate receptors.13 The mechanism base of support of hind paws as compared with control rats responsible for the long-term maintenance of a low extra- (Figure 2C). In addition, hyponatremic rats demonstrated a cellular glutamate concentration is astrocytic glutamate significantly larger duty cycle for front paws (Figure 2D) uptake, in which the sodium-dependent glial glutamate reflecting a significantly smaller support two gait phase (the transporters, GLT-1 and GLAST, play a critical role.13 Con- relative duration of simultaneous contact with the glass plate sidering that chronic hyponatremia reduces the brain’sglu- of two paws) and a significantly larger support three plus four tamate content by about 40%, it is possible that chronic gait phase (that of three or four paws) (Figure 2E). These hyponatremia affects glial glutamate uptake and glutamate findings suggest that chronic hyponatremia causes ataxic gait. metabolism. In this study, we developed a syndrome of inappropriate Chronic Hyponatremia Causes Memory Impairment antidiuretic hormone secretion rat model with different serum We evaluated the effect of chronic hyponatremia on recogni- sodium ion concentrations ([Na+]). Then, we showed that a tion memory using a novel-object recognition test (NORT). reduction in serum [Na+] in chronic hyponatremia induces Therewerenosignificant differences in exploratory times gait disturbances, memory impairment and decreased long- among the three groups (Figure 3A). During training sessions, term potentiation (LTP) at hippocampal CA3–CA1 synapses all three groups spent approximately equal time with each as an underlying mechanism of memory impairment. Fur- object (Figure 3B). However, during retention sessions, the thermore, the extracellular glutamate